Blood platelets, upon activation by extracellular agents such as thrombin or collagen, undergo a complex series of physiological reactions including shape change, aggregation, and secretion. The extracellular agents interact with specific receptors on the external surface of the platelet plasma membrane. Although the biochemical events linking receptor occupancy and the physiological response of platelets are not clear, it is known that receptor occupancy results in an increase of intracellular free1Ca2+. The elevated Ca2+ is likely acting as an intracellular signal, triggering the biochemical reactions necessary for the physiological response. Calmodulin, a ubiquitous Ca2+ binding protein, mediates the Ca2+ regulation of many enzymes and cellular processes in eukaryotic organisms. Preliminary evidence suggests that calmodulin is the principal receptor for the Ca2+ signal within the platelet. This research project will investigate in a systematic manner the role of calmodulin in reactions which are likely to be important to the physiological functions of platelets. The following questions will be addressed: (1) What is the concentration of calmodulin in the platelet and its subcellular distribution; (2) which platelet soluble proteins bind to calmodulin in a Ca2+ dependent manner; (3) which platelet enzymes are regulated by Ca2+ through the mediation of calmodulin; (4) do the platelet plasma and granule membranes contain calmodulin-binding proteins, and (5) what are the identity of the membrane calmodulin-binding proteins? By asking such basic questions concerning the role of calmodulin in platelet, we are likely to gain new insight into the biochemical mechanisms by which the intracellular CA2+ signal elicits a physiological response in the platelet. This is important; platelets play a central role in the maintenance of hemostasis as well as the pathological conditions of thrombosis and atherosclerosis. Understanding the basic biochemical mechanisms involved in platelet function may provide a rationale for developing new pharmacological agents which will allow the manipulation of these physiological and pathological conditions.